1. Field of the Invention
The present invention relates to a modulating method adapted for use in digitally recording data on a recording medium and, more particularly, in determining the optimal range of a minimum run length d (minimum number of successive same symbols) in a variable-length code (d, k; m, n; r).
2. Description of the Related Art
In a magnetic recording system for example, generally its signal frequency characteristic is differential, and there occurs deterioration of the characteristic in a higher frequency range of the signal. Such deterioration is derived from, e.g., a variety of losses relative to a head gap, the space between a head and a recording medium, the thickness of a recording medium, and low-frequency loss caused by a rotary transformer. In addition, random errors are induced by crosstalk noise from adjacent tracks, noise from a recording medium, or overwrite noise. For achieving an exact operation of recording and reproducing data despite any of such losses and noises, it is preferred that the digital data is modulated, prior to being recorded on a medium, in a manner to be adequate for the recording system, whereby a greater amount of the information can be recorded with stability. For the purpose of meeting the above requirement, it is generally customary to channel-code the data in accordance with predetermined rules.
A block code is included in such channel code. The block code is used for forming a train of data into blocks each consisting of m.times.i bits, and converting the data words into a recording code of n.times.i channel bits in accordance with adequate coding rules. When i is equal to 1, it becomes a fixed-length code; whereas when i and the maximum restriction length r are both greater than 1, it becomes a variable-length code. This block code is also termed (d, k; m, n; r) code, where d stands for the minimum number of successive same symbols (e.g., 0's in the case where date is encoded such that a 1 represents a transition, and a 0 represents no transition; thus a 0 in the encoded signal represents the fact that the same symbol in the unencoded form as the previous symbol is written or read); k for the maximum number of successive same symbols (0's in the transition encoded form); m for the length of a basic data word; and n for the length of a basic code word.
In compact discs for example, there is adopted an eight-to-fourteen modulation (EFM) system. In the EFM code where d is 2, its minimum inversion interval Tmin is 1.41, and its detection window width Tw is 0.4.
The recording density in a compact disc can be increased by reducing the lengths of pits on the disc. However, if the pit length is excessively reduced to be smaller than the diameter of a detecting laser beam, there arise difficulties in performing proper detection of the pits. Therefore, in case the pit length is reduced, the wavelength of the detecting laser beam needs to be shorter. As listed in Table 1, the wavelength of a red laser beam is 780 nm, while that of a green laser beam is 532 nm. And if a blue laser is realized, its wavelength is as short as 420 nm. As compared with the reference surface density in a red laser, the surface density attained by the use of a green laser or a blue laser can be increased 2.15 times or 3.0 times.
TABLE 1 ______________________________________ Red laser Green laser Blue laser ______________________________________ Wavelength 780 nm 532 nm 420 nm Surface 1.0 times 2.15 times 3.0 times density NA: 2.0 times 4.3 times 6.0 times 0.4 .fwdarw. 0.6 ______________________________________
In addition, if the numerical aperture NA of an objective lens for focusing a laser beam is increased from a normal value of 0.4 to 0.6 for example, it becomes possible to enhance the surface density 2.0, 4.3 or 6.0 times in comparison with the value at the NA of 0.4 in a red, green or blue laser.
However, for recording video information (moving pictures) at a density of 10 Mbps on a disc equal in diameter (12 cm) to a compact disc, it is necessary to increase the density more than 6 times since the linear density in the conventional compact disc is approximately 1.5 Mbps. And there exist difficulties in realizing such a high density even by the use of a green laser, as is obvious from Table 1.